1. Field of the Invention
The present invention relates to an illumination optical system for endoscopes.
2. Description of the Prior Art
In recent years, it has been demanded to widen field angles of illumination optical systems for endoscopes under a trend where objective optical systems for endoscopes have wider field angles. Further, it has been demanded to develop illumination optical systems which illuminate objects to be observed with adequate luminance distributions.
As a conventional example of the illumination optical systems for endoscopes which respond to the demands described above, there is known an optical system disclosed by Japanese Patent Kokai Publication No. Sho 56-20,428. This illumination optical system has a composition as illustrated in FIG. 1, wherein a positive lens system 2 is disposed before a light guide 1 consisting of an optical fiber bundle so that a light bundle emerging from the light guide is once converged and then diverged for illumination at a wide field angle.
When a ray that emerges from the light guide 1 in parallel with an optical axis of the light guide 1 is incident on the lens system 2 at a height of h and the ray having the height h of incidence emerges from the lens system 2 at an angle of .theta., this conventional illumination optical system is configured so as to have a relationship between the height h of incidence and the angle of emergence .theta. expressed by the following formula: EQU h=f.cndot.sin.theta.
wherein the reference symbol f represents a focal length of the illumination optical system.
A distribution of relative luminance which is obtained by illuminating a planar object with this conventional illumination optical system is determined as described below. When a surface of an object which is perpendicular to an optical axis of an optical system and a completely diffusing planar surface is illuminated by rays which are emitted from a light source and have passed through the optical system, relative luminance on the surface which is set in a direction at an angle of .theta. relative to the optical axis is expressed by the following formula (i): EQU F(.theta.)=(.beta./.beta..sub.M .times..beta./.beta..sub.S).sup.-1. . . (i)
wherein the reference symbol .beta. represents a paraxial magnification for the surface of the object, and the reference symbols .beta..sub.M and .beta..sub.S designate magnifications in the meridional direction and the sagittal direction respectively relative to the surface of the object.
When the object is located sufficiently far from an exit pupil of the lens system, .beta..sub.M and .beta..sub.S used in the above-mentioned formula (i) are given by the following formulae (ii) and (iii) respectively: EQU .beta..sub.M =.beta. cos.sup.2 .theta.{dA(.theta.)/d.theta.}. . . (ii) EQU .beta..sub.S =.beta.{A(.theta.)/tan .theta.} . . . (iii)
wherein A(.theta.)=h/f.
From the above-mentioned formulae (ii) and (iii), it will be understood that, when the planar object having the completely diffusing surface is illuminated through the above-described illumination optical system, the distribution of relative luminance is F(.theta.)=cos.sup.4 .theta. and the luminance is lowered from a center toward marginal portions of the object in proportion to cos.sup.4 .theta. as illustrated in FIG. 2. In FIG. 2, the ordinate represents luminance on the surface of the object and the abscissa designates the angle of emergence of the ray from the lens system described above.
When a surface of a spherical object or a tubular object is illuminated by rays which have passed through this conventional illumination optical system, a distribution of relative luminance can be determined as described below: A distribution of relative luminance on a completely diffusing surface of the spherical object and a distribution of relative luminance on a completely diffusing surface of the tubular object are generally given by the following formulae (v) and (vi) respectively: EQU G(.theta.)=F(.theta.).times..sup.1 /cos.sup.3 .theta. . . . (v) EQU H(.theta.)=F(.theta.).times.tan.sup.3 .theta. . . . (vi)
wherein the reference symbols G(.theta.) and H(.theta.) represent distributions of relative luminance on the completely diffusing surfaces of the spherical object and the tubular object respectively.
From the above-mentioned formulae (v) and (vi), it will be understood that the distribution of relative luminance on the completely diffusing surface of the spherical object and that on the completely diffusing surface of the tubular object are expressed as G(.theta.)=cos .theta. and H(.theta.)=cos .theta. sin.sup.3 .theta. respectively, or as illustrated in FIG. 2.
As is apparent from FIG. 2, the luminance on the completely diffusing surface of the spherical object is lowered from the center toward the marginal portion thereof, but the distribution of relative luminance poses no problem for practical use, whereas the distribution of relative luminance on the completely diffusing surface of the tubular object is not enhanced abruptly at marginal portions of a visual field and adequate.
In case of an illumination optical system which satisfies h=f sin .theta. like the above-described conventional illumination optical system, a second surface and a third surface, as counted from the object side, of the illumination optical system must have strong refractive powers when this optical system is to be configured so as to have a wide field angle for observing objects through an observation optical system which has a wide field angle of 110.degree. or larger. When the second surface and the third surface have strong refractive powers, h is not proportional to sin .theta., and rays having large heights are totally reflected by a first surface and the third surface of the illumination optical system. Further, the rays are more apt to be totally reflected as these rays have larger heights of incidence and luminance is lowered abruptly within a region exceeding the wide field angle of 110.degree..
Furthermore, there is known, as an example of illumination optical systems, which are usable in combination with observation optical systems having field angles of 110.degree. and larger, an optical system disclosed by Japanese Patent Kokai Publication No. Sho 58-95,706. This optical system has a composition illustrated in FIG. 3 and requires a manufacturing cost which is higher than that of the conventional optical system shown in FIG. 1 since the former comprises lens components in a number larger than that of the lens components comprised in the latter.
Moreover, there is known, as an illumination optical system which can illuminate planar objects with a uniform distribution of luminance, there is known an optical system in which the height h of incidence is nearly proportional to tangent of the angle of emergence .theta.. This is an illumination optical system disclosed by Japanese Patent Kokai Publication No. Sho 62-178,207 which has a composition illustrated in FIG. 4. However, objects to be observed through endoscopes are not only planar objects but also spherical objects, tubular objects and other various objects having different forms. Objects to be observed through medical endoscopes are, for example, spherical objects such as interiors of stomachs, and tubular objects such as alimentary canals and bronchi.
When a spherical object is illuminated with a light bundle having passed through the illumination optical system in which h is proportional to tan .theta., it will be understood from the formulae (i) and (v) that luminance is enhanced from a center toward a marginal portion in proportion to 1/cos.sup.3 .theta. as indicated by G(.theta.) in FIG. 5. In an actual illumination system, however, rays which are to travel toward the marginal portions of a visual field are irregularly and totally reflected by the inside of the outer circumferential surface of a lens component, whereby these rays are not used for illumination. Accordingly, luminance is lowered abruptly as indicated by the curve b in FIG. 6. Consequently, the distribution of luminance on the spherical surface has ring-shaped non-uniformities. In FIG. 6, the curves a, b and c represent distributions of luminance on the planar surface, the spherical surface and the tubular surface respectively.
When the tubular surface is illuminated with a light bundle which has passed through this conventional illumination optical system, luminance is enhanced abruptly toward the marginal portion in proportion to tan.sup.3 .theta. as is understood from the formulae (i) and (vi).
This conventional illumination optical system provides only a narrow range within which objects can be observed with adequate brightness and is not desirable for practical use.